Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light. Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes and, in some cases, significantly higher efficiency for converting electric energy to light.
For the purposes of this discussion, an LED can be viewed as having three layers, the active layer sandwiched between two other layers. The active layer emits light when holes and electrons from the outer layers combine in the active layer. The holes and electrons are generated by passing a current through the LED. The LED is powered through an electrode that overlies the top layer and a contact that provides an electrical connection to the bottom layer.
The cost of LEDs and the power conversion efficiency are important factors in determining the rate at which this new technology will replace conventional light sources and be utilized in high power applications. The conversion efficiency of an LED is defined to be the ratio of optical power emitted by the LED in the desired region of the optical spectrum to the electrical power dissipated by the light source. The electrical power that is dissipated depends on the conversion efficiency of the LEDs and the power lost by the circuitry that converts AC power to a DC source that can be used to directly power the LED dies. Electrical power that is not converted to light that leaves the LED is converted to heat that raises the temperature of the LED. Heat dissipation often places a limit on the power level at which an LED operates. In addition, the conversion efficiency of the LED decreases with increasing current; hence, while increasing the light output of an LED by increasing the current increases the total light output, the electrical conversion efficiency is decreased by this strategy. In addition, the lifetime of the LED is also decreased by operation at high currents.
Single LED light sources are not capable of generating sufficient light to replace conventional light sources for many applications. In general, there is a limit to the light per unit area of LED that can be practically generated at an acceptable power conversion efficiency. This limit is imposed by the power dissipation and the electrical conversion efficiency of the LED material system. Hence, to provide a higher intensity single LED source, larger area chips must be utilized; however, there is a limit to the size of a single LED chip that is imposed by the fabrication process used to make the LEDs. As the chip size increases, the yield of chips decreases, and hence, the cost per LED increases faster than the increase in light output once the chip size increases beyond a predetermined size.
Hence, for many applications, an LED-based light source must utilize multiple LEDs to provide the desired light output. For example, to replace a 100-watt incandescent bulb for use in conventional lighting applications, approximately 25 LEDs having chips of the order of 1 mm2 are required. This number can vary depending on the color temperature desired and the exact size of the chips.
In addition, the light source typically includes a power supply that converts either 115V or 240V AC power to a DC level compatible with driving the LEDs. The conversion efficiency of this power supply, often 80% or less in cost-competitive products, also contributes to the overall power-to-light conversion efficiency of the light source. To provide the maximum power delivery efficiency, the output of the power supply should be near the peak voltage of the AC power, and the current that must be delivered across the various conductors in the light source should be minimized to avoid resistive losses in the conductors. A typical GaN LED requires a drive voltage of about 3.2-3.6V. Hence, from a power conversion efficiency point of view, the 25 LED light source described above would be constructed as a single string of 25 LEDs connected in series with an output voltage level from the power supply of approximately 80 volts.
However, there are other considerations such as the cost and reliability of the light source that must be taken into consideration in addition to the power-to-light conversion efficiency. From a reliability point of view, a single series connected string of LEDs is the poorest option. In general, LEDs are more likely to fail by forming an open circuit than a short circuit. For example, a wire-bond that connects a pad in the LED to external circuitry can fail. Hence, a single LED failure in the series-connected string leads to the catastrophic failure of the light source.
From a reliability point of view, a light source in which all of the LEDs are connected in parallel would appear to be the best if the predominant failure mechanism is LEDs failing by forming open circuits. If a single LED fails and a constant current source is used to drive the parallel connected LEDs, the current through the other LEDs will increase slightly, and hence, the other LEDs will partially compensate for the light lost when one of the LEDs fails. Unfortunately, such an arrangement is inefficient from the point of view of the power supply efficiency and requires conductors that can handle very large currents without introducing significant transmission costs.
In addition to reliability and power conversion efficiency, the designer must provide a design that can accommodate the variations in the light generation efficiency among individual LEDs. LEDs are fabricated on wafers that have some degree of non-uniformity across the wafer as well as variations from wafer to wafer. As a result, the amount of light generated by commercially available LEDs has a significant variation from LED to LED. The allowable variation in the light output of the final light source is determined by the need to have light sources that all generate the same amount of light and have the same appearance. In general, the variation in light output among the LEDs is too great to meet the needs of the light source manufacturers without some sorting of the LEDs to provide LEDs with less variability. The sorting process adds to the costs of the light source. In addition, many light sources cannot utilize LEDs that are not within a range of intensities that is less than the spread in intensities of the LEDs as manufactured. As a result, there is less of a market for LEDs that are not in the range of interest, which increases the cost of the LEDs in the desired range and decreases the cost of the LEDs that are outside that range.
The problems inherent in balancing reliability against power supply efficiency are reduced by constructing light sources in which a plurality of component light sources are connected in parallel. Each component light source consists of a plurality of LEDs connected in series, and hence, utilizes a significantly higher driving voltage than the individual LEDs. For example, a typical GaN LED requires a drive voltage of about 3.2 volts and a current of 0.35 amps. To provide a light source having approximately 2000 lumens, 25 such LEDs must be driven. The light source can be constructed by connecting 5 component light sources in parallel. Each component light source consists of 5 LEDs connected in series. Hence, the driving voltage is improved by a factor of 5 to 16V. If one LED fails by becoming an open circuit, the remaining 4 component light sources still function, and hence, the light source continues to function, albeit at a reduced brightness. However, the remaining LEDs are overdriven by 20 percent since these LEDs must pass the current that no longer passes through the open circuited component light source. As a result, the lifetimes of the remaining LEDs are significantly shortened.
Unfortunately, this strategy does not eliminate the need for utilizing only a subset of the production run of LEDs for any given final light source.